Hey EleftheriadisStauros,
From the screenshot you provided, it looks like this happens after choosing your user account and entering your password. If that's right, there could be a login item keeping your Mac from reaching the Desktop. To find out try starting your Mac to safe mode. Safe mode can help as it completes several tasks during start up and prevents certain software from loading. Here's a guide with more details about safe mode, how to start there, and what to do after: How to use safe mode on your Mac - Apple Support
After starting to safe mode, depending on the results, you'll either continue with the steps under "If the issue continues in safe mode" or "If the issue doesn't continue in safe mode".
If that doesn't help, let us know more details:
- Which Mac model do you have exactly? We're looking to know if it has Apple silicon or an Intel processor as the troubleshooting steps are different.
- What happens when trying to start to macOS Recovery? Does an alert appear, and if so, what does it say?
Let us know what you find.
Take care!